The present invention relates to a microwave burning furnace for burning an object to be burned which is formed by the material of a pottery or fine ceramics material, thereby manufacturing a burned product.
In recent years, it has been proposed to burn the material of a pottery or fine ceramics by microwave heating, and practical use has already been started.
In the case in which they are to be burned by the microwave heating, a microwave uniformly heats each portion of an object to be burned in principle if the object to be burned is homogeneous. In a burning process, however, an atmospheric temperature in a microwave burning furnace is much lower than the surface temperature of the object to be burned. For this reason, a heat is radiated from the surface of the object to be burned. As a result, a temperature gradient is generated between a central part and a surface in the object to be burned so that a crack is apt to be generated.
Referring to the characteristic of the microwave heating, furthermore, the dielectric loss of the same substance is increased when a temperature is higher. If the temperature gradient is once generated, accordingly, the microwave absorption efficiency of a portion having a high temperature is high and a difference in the microwave absorption efficiency is further increased so that partial and local heating is caused.
If the temperature gradient is once generated, thus, a difference in a temperature is more increased by the microwave heating. Consequently, the generation of the crack is promoted.
In the burning to be carried out by the microwave heating, moreover, there is also a problem in that an energy effect is obtained poorly by the microwave heating through temperature rising in a low temperature region in the case in which the material of the object to be burned uses, as a raw material, alumina or silica which is the main material of ceramics having a small dielectric loss at an ordinary temperature.
As a microwave burning furnace capable of suppressing the generation of the temperature gradient to reduce the generation of a crack, there has been proposed a microwave sintering furnace (see JP-A-6-345541) provided with a heater 24 in the microwave sintering furnace and serving to control a temperature in a microwave burning furnace by means of the heater 24 as shown in FIG. 11.
The microwave burning for ceramic can have various forms, for example, a form for burning an object to be burned such as ceramic through self-heat generation by a microwave and a form in which a heating unit for generating a heat by a microwave is provided close to an object to be burned and burning the object to be burned by the heat of the heating unit. The burning furnace according to JP-A-6-345541 takes the former form.
As a microwave burning furnace having the latter form, moreover, there has also been proposed a structure in which a peripheral wall is formed by a heating unit for carrying out self-heat generation through a microwave in a furnace as shown in FIG. 12 (see JP-A-2-275777). The burning furnace serves to accommodate a cylindrical vessel 25 formed by a microwave transmitting heat insulator in a microwave oven and to provide a cylindrical member 26 formed by a silicon carbide sintered body in the vessel 25, to set the inside of the cylindrical member 26 to be a sintering portion 27, to put an object to be burned therein, and to irradiate a microwave to cause the silicon carbide sintered body to generate a heat, thereby burning the object to be burned.
As a form using both of the forms together, there has been proposed a burning furnace (see JP-A-7-318262) having a heat generating vessel containing a substance having a great microwave loss as a main component and a heat insulator serving to cover the outside of the heat generating vessel and containing a substance having a small microwave loss as a main component, the heat generating vessel being provided with an opening, and furthermore, having a microwave irradiating device for irradiating a microwave toward the heat generating vessel through the heat insulator and irradiating the microwave toward an object to be burned in the heat generating vessel through the opening of the heat generating vessel. This can relieve a temperature distribution in the direction of a thickness.
Furthermore, there has been proposed a burning furnace (see JP-A-2002-130960) comprising a burning chamber 56 divided to surround the whole periphery of an object 50 to be burned by means of a blanket 55 capable of automatically generating a heat by a microwave, and microwave generating means 52 for irradiating a microwave on the object to be burned which is provided in the burning chamber 56, wherein the amount of heat generation per unit volume of the blanket 55 through the microwave is larger than the amount of heat generation per unit volume of the object to be burned and a surface temperature in the blanket 55 and that of the object 50 to be burned are substantially equal to each other as shown in FIG. 13.
It was found that the object to be burned can be heat insulated falsely and completely by perfectly surrounding the periphery of the object to be burned through a blanket having a microwave absorbing characteristic which is equivalent to the object to be burned in the burning through the microwave. In this case, it was supposed that the generation of a thermal gradient on the object to be burned by radiant cooling can be suppressed and the burning can be carried out still more uniformly. In the case in which the object to be burned is surrounded by the blanket and is thus burned, the energy of the microwave is also absorbed into the blanket as well as the object to be burned and is thus consumed. For this reason, there is a problem in that the amount of an energy required for the burning is increased considerably.
In order to reduce the amount of the energy consumed by the blanket, the amount of a thermal energy lost from the blanket toward an outside is larger than that given to the blanket through the microwave if the thickness of the blanket is reduced. For this reason, a great difference in a temperature is made between the inner surface of the blanket and the object to be burned. In order to solve the problem, therefore, there has been proposed a burning furnace capable of suppressing the generation of the thermal gradient over the object to be burned by the radiant cooling while reducing the amount of the energy required for the burning of the object to be burned.
The problem is solved by means in which the amount of heat generation per unit volume of the blanket through the microwave is larger than that per unit volume of the object to be burned and the surface temperature of the inside of the blanket and the surface temperature of the object to be burned are substantially equal to each other.
In the structure in which the heater 24 capable of independently executing a heat treatment is additionally provided as in the microwave burning furnace in JP-A-6-345541, the temperature rising in a low temperature region which is hard to perform through the microwave heating is compensated by heating through the heater 24 so that it is also possible to carry out the burning for the object to be burned having a small dielectric loss at an ordinary temperature. Thus, it is possible to improve an energy efficiency required for the burning.
By covering the blanket dividing the burning chamber with another blanket having an excellent heat insulating property as described in JP-A-2002-130960, moreover, it is possible to enhance the heat insulating property around the burning chamber and to suppress the generation of a temperature gradient due to heat radiation.
In the technique in each of the Documents, however, there is a problem in that the structure of the microwave burning furnace is complicated and a cost is thus increased.
More specifically, in case of the technique in JP-A-6-345541 in which the heater is additionally provided, the heater is exposed to a burning temperature region having a very high temperature. For this reason, it is necessary to cause the heater to have a very high heat resistance. Consequently, it is impossible to employ a sheath type heater which is comparatively inexpensive and has an excellent performance. Moreover, a lead wire for supplying a power to the heater penetrates through a cavity. Therefore, it is necessary to employ a microwave sealing structure for sealing the leakage of a microwave in the penetrating portion, and furthermore, the insulating treatment of the lead wire and the cavity. Furthermore, there is also a problem in that power supplies having large capacities are to be mounted for supplying a power for a microwave generator and a heater respectively. These problems cause a drawback that the structure is complicated and the cost is increased.
In case of JP-A-2002-130960, moreover, some advantages can be obtained for suppressing the generation of the temperature gradient and there is also a problem in that an effect for an improvement in an energy efficiency in the temperature rising in the low temperature region is poor.